It has been a matter of common knowledge heretofore that oxygen contained in a large amount not only deteriorates sinterability but also gives rise to the growth of minute holes in the sintered hard metal thereby reducing the toughness thereof.
West German Laying-Open Gazette No. 2043411 teaches us that oxygen contained in the sintered hard metal should be strictly less than 0.15 wt %.
In "modified Spinodal Alloys for Tools and Wear Applications, 8th Plansee Seminar II (1974)" by Rudy et al, it is reported that oxygen contained up to 2.5 wt % as a sintered hard metal component does not deteriorate sinterability, but fracture toughness is reduced and no dense phases are obtainable if its content is more than 0.5 and 0.9 wt % in case of a single .alpha.' phase (a carbonitride phase having a small amount of Mo) and .alpha." phase (a carbonitride phase having a large amount of Mo), respectively.
The method of Rudy et al is characterized in that carbonitride alloy powder (TiMo) (CN) is used as raw material.
Though the method of Rudy et al has improved the conventional method to a certain extent, there is no change in the fundamental phenomenon of the discharge of the contained oxygen, whereby the toughness of the sintered hard metal is reduced.
Thus, according to the method of Rudy et al, oxygen contained in the sintered hard metal is not stabilized and liable to be discharged as CO or CO.sub.2 gas thereby reducing the toughness of the sintered hard metal. After all, it has been a conventional conception that is difficult to cause a sintered hard metal to contain oxygen therein with stability.